Accurate determination of microwave material permittivity and loss factor over the entire microwave regime and over the desired temperature range of operation are needed for accurate design, operation and evaluation of microwave components, circuits, antenna and systems. Microwave engineers can input such precise material parameters into currently available software programs to accurately model devices as functions of temperature. In this way, the number of iterations and time required to develop components, circuits, subsystems and systems that operate to the specified performance level and over the specified temperature range of operation can be reduced. Such efficient development, rather than development by trial and error, will lower component, circuit, subsystem and system cost.
Many methods have been devised for the accurate determination of microwave permittivity and loss factor, however, most of these methods employ cavity resonators or length of waveguide that are typically limited to measurement at one or only a few discrete frequencies and typically only one temperature, usually room temperature. Further, such methods typically become increasingly difficult to apply at higher frequencies and when evaluating high permittivity materials as the wavelength of radiation decreases. For solid dielectric materials a major problem is the accurate fabrication of a specimen to fit closely into the resonator or waveguide. Fabrication and measurement of ceramic samples can be especially difficult. Small air gaps between the dielectric and the metal resonator wall can cause large measurement errors with the magnitude of the error increasing with decreasing wavelength since the dimensions of the specimen are inversely proportional to the wavelength. At higher frequencies, it can be difficult to precisely determine the loss tangent of a material using the resonant cavity method if the sample's loss tangent is small. This occurs when the loss tangent falls below the reciprocal of the Q factor of the empty resonant cavity and occurs because Q factor varies as fO−3/2.
At the present time, due to the limits of typical resonant cavity methods, microwave engineers have limited information, typically data for only one temperature and a few discrete frequencies, about the dielectric constant and loss tangent of many technologically important microwave materials for use as input parameters for device design and development. Thus, it is important to develop versatile microwave methods that can be used to obtain comprehensive information of the dielectric constant and loss tangent of materials over a wide range of frequencies and temperatures. With such information, the microwave engineer will be able to more accurately and rapidly design and develop components, circuits, sub-systems and systems that meet performance specifications over the desired temperature range thus reducing cost.
An extremely powerful measurement method, known as the millimeter wave Fabry-Perot interferometer, for the determination of permitivity and loss factor has previously been demonstrated. An advantage of the confocal resonator or Fabry-Perot type interferometer is the ability to utilize the relatively high Q TEM00p modes for determination of permittivity and loss factor at numerous discrete frequency intervals which is dependent upon the plate separation distance and mode number as documented both theoretically and experimentally. The cavity is an open cavity resonator having opposed mirrors and a frame for holding a sample of the material, which is insertable into the cavity between the mirrors. The sample requires no electrical contacts and only needs to be placed uniformly in the beam. Microwave energy is transmitted via the mirrors, one to the other, and readings are taken, first without a sample and then with a sample, to obtain values of the desired properties.
Open confocal resonator cavities are commercially available; however, such systems have several severe limitations for precision determination of microwave permittivity and loss factor. The major drawbacks are (1) a fixed cavity length, L, that is preset by the manufacturer, (2) inadequately designed sample holder, and, (3) no ability for temperature variation. Because the resonant frequencies depend upon cavity length, a fixed cavity length limits the measurement frequencies to predetermined fixed discrete values. As discussed in more detail later, this problem is overcome by the present invention by placing the cavity resonator plates on micrometer drives that can be driven so as to vary the distance between the plates and hence the discrete resonant frequencies of the cavity.
The inadequately designed sample holder can be the source of relatively large errors and non-repeatability of measurements. This problem is overcome herein by placing the sample on an appropriately designed stage that allows precision sample alignment and repeatable sample insertion. The problem of the inability to vary temperature of the sample is also overcome by placing the whole instrument within an environmental chamber.